The present invention relates to hydraulic shock absorbers with variable damping resistance or force, especially for use in vehicle suspension systems.
There has been proposed a hydraulic shock absorber for use in vehicle suspension systems, designed to adjust damping resistance so as to improve riding comfort and steerability in response to road and running conditions.
A typical hydraulic shock absorber includes a piston assembly within which a primary oil passage is defined, as disclosed in Japanese laid-open utility model publication No. Sho 62-167950. A pressure chamber is defined downstream of a main valve as a damper mechanism and communicated with upstream and downstream cylinder chambers through fixed and variable orifices, respectively.
In such a conventional hydraulic shock absorber, the variable orifice is opened and closed so as to adjust the effective cross sectional area of a fluid path between the upstream and downstream cylinder chambers and also, adjust pressure within the pressure chamber necessary to open the main valve. During adjustment of the variable orifice, damping resistance is substantially proportional to the square of piston speed. During adjustment of the pressure within the pressure chamber, damping resistance is substantially proportional to piston speed. Thus, the related art shock absorber is able to achieve a wide range of damping adjustment.
The pressure chamber is defined by slidably fitting the main valve around a cylindrical valve guide. However, this arrangement causes oil leakage from sliding parts between the main valve and the valve guide. The viscosity of oil is varied depending on temperature and has an influence on oil leakage. In other words, damping resistance is varied in response to a change in temperature. Moreover, the related art shock absorber is expensive to manufacture since the sliding parts must be machined with high accuracy.
In order to solve the problems encountered in the related art shock absorber, the inventors of the present application previously proposed a hydraulic shock absorber wherein a disc valve, a retainer disc, a seal disc and other discs are arranged in a stack whereby a partition is formed between a main valve and a pressure chamber to prevent oil leakage, as disclosed by Japanese patent application No. Hei 8-283114. The inner periphery of the retainer disc is clamped or secured to an inner seal ridge of a valve body through the disc valve. In other words, the entire surface of the retainer disc is in surface contact with the disc valve. This arrangement results in an increase in frictional resistance between the disc valve and the retainer disc when the disc valve is opened against the bias of the retainer disc. Such an increase in frictional resistance causes unstable damping resistance.
Accordingly, it is an object of the present invention to provide a hydraulic shock absorber which provides a wide range of damping adjustment and gives stable damping resistance.